A twenty-foot piece of wire is cut into three pieces. The first piece is twice as long as the second piece, and four times as long as the third piece. How long is the second piece of wire?

Answer in the book

The equation 2x+x+1/2x=20 modes this situation. Therefore, 3 1/2x=20, x= 5 5/7 feet

I understand how they got 2x+x, but don't understand how they got 1/2x. Can someone explain it?

Thanks in advance.